Global ConfigParser

[benthayer]

fixes #775

We can instantiate a GitConfigParser with no files specified, if we do so, then it'll figure out where those files are. It does this by either having a config_level specified to it or, in the case of a config reader, by default we might want to read from all configuration levels, just like is currently the case in Repo.config_writer(), so we iterate through all of them (except repository level) and add all files to the parser.

If the repository level is specified, we raise a unique ValueError telling the user to use the Repo object instead.

[codecov-io]

Codecov Report

Merging #950 into master will decrease coverage by 0.01%.
The diff coverage is 22.72%.

@@            Coverage Diff            @@
##           master    #950      +/-   ##
=========================================
- Coverage   93.52%   93.5%   -0.02%     
=========================================
  Files          59      59              
  Lines        9677    9915     +238     
=========================================
+ Hits         9050    9271     +221     
- Misses        627     644      +17

Impacted Files	Coverage Δ
git/config.py	88.4% <22.72%> (-3.87%)	⬇️
git/objects/blob.py	100% <0%> (ø)	⬆️
git/refs/remote.py	100% <0%> (ø)	⬆️
git/test/test_diff.py	100% <0%> (ø)	⬆️
git/test/performance/test_streams.py	100% <0%> (ø)	⬆️
git/test/test_tree.py	100% <0%> (ø)	⬆️
git/test/test_exc.py	100% <0%> (ø)	⬆️
git/test/test_docs.py	99.6% <0%> (ø)	⬆️
git/test/test_refs.py	99.18% <0%> (ø)	⬆️
git/test/test_config.py	98.8% <0%> (ø)	⬆️
... and 27 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dfa0eac...d6c4415. Read the comment docs.

[Byron]

Thanks a lot for your contribution, it's looking great! I particularly like that the change is implemented without breaking compatibility.

There are just two minor things I saw when reviewing, please let me know what you think about them.

[Byron]

Thanks a lot for your effort! It's a great improvement!

[benthayer]

Happy to help!